From Daily Kos:
DS: What is Dark Matter?
SC: “Dark” is a euphemism — it means not only that the stuff is completely invisible, but that it isn’t anything ever seen in a laboratory here on Earth. Clearly, we’d rather not have to invoke such stuff. Nevertheless, the data have forced us to believe that ordinary matter is only about 5% of the universe; another 25% is “dark matter,” and the remaining 70% is “dark energy.”
Dark matter is some kind of particle that doesn’t interact with light, so that we can’t see it directly. We know it’s there because it creates a gravitational field, and we can detect that gravitational field. In fact, we detect it over and over again — the single idea of dark matter allows us to account for the behavior of our Milky Way, of other galaxies, of large-scale groups of galaxies, of the expansion of the universe, and of the patterns we observe in the microwave background. The most popular dark-matter candidates are “weakly-interacting massive particles” (or WIMPs), which are predicted by models such as “supersymmetry.” Supersymmetry is an ambitious idea that proposes a new kind of fermion (matter-like particle) for every existing boson (force-like particle), and vice-versa. The Large Hadron Collider in Geneva, scheduled to turn on next year, will be looking for supersymmetry, among other things.